Questions

How can I find companies in a certain niche?

I'm trying to find online services like kenhub.com. They offer a subscription based online learning platform for medical students to study anatomy. I'm not looking for a similar service but for companies that have this business model.
Subscription based - online learning platforms - focusing on a certain topic.

I would dig into some Google search results for a few hours. Start with the example above and search "learn anatomy online" which will show you top sites and competitors' ads for sites like kenhub.com. Then broaden your search and use Google's autocomplete results to get ideas for more industries. Start typing "online learning for..." and you'll see suggested searches. Also on the bottom or right side of search results pages, you'll see related searches and "people also searched for" which will continue expanding your search paths. Once you gather a list of sites that fit your criteria you can use a tool like www.spyfu.com to find their main competitors. Set a timer because you can get lost in this kind of research for days!!

Well, the simplest answer is to use Google, because the organic search can be really helpful. But you can also visit such websites as Clutch, GoodFirms, etc, where you can choose required features (location, hourly rate, etc.).